"Stories out of Europe in reference to the ECB keep coming out, but market players are probably tired of running on rumors and want to see the “real deal” before making commitments for more than a few hours," wrote Elliot Spar, senior vice president at Stifel Nicolaus, in his daily note.

The Dow Jones Industrial Average eked out a gain of 11.54 points, or 0.09 percent, to close at 13,047.48, led by Disney.

The S&P 500 erased 1.50 points, or 0.11 percent, to finish at 1,403.44. The Nasdaq slipped 5.79 points, or 0.19 percent, to end at 3,069.27.

The CBOE Volatility Index, widely considered the best gauge of fear in the market, ended below 18.

Among the key S&P sectors, materials gained, while utilities lagged.

Traders will be paying close attention to the ECB meeting on Thursday at which president Mario Draghi is expected to announce a bond-buying program to help countries such as Spain and Italy lower their borrowing costs. European stocks initially saw a small boost after a report that Draghi would propose unlimited bond buying at the meeting. But an ECB spokesman declined to comment on the report. (Read More: Fed Watching ECB Just as Closely as Markets Are)

“Everyone’s looking ahead to the ECB, but until the German high court's [decision on September] 12, the ECB’s going to be handcuffed,” said Kenny Polcari, managing director of ICAP Equities. “Tomorrow, the ECB might [come out with] a program that announces some bond buying but it’s not going to be the big bazooka that [Draghi] promised, so there’s a potential that the market’s going to be disappointed.”

Meanwhile, a survey showed that the euro zone is likely to have slipped back into recessionin the current quarter, with Germany's composite PMI indicating the lowest reading since June 2009.

S&P 500's Next Stop: 1,600?

The FMHR traders weigh in with the trade on transports. Also, discussing why the S&P 500 could hit 1,600 by the end of the year, with Barry Bannister, Stifel Nicolaus strategist.

“Most investors appear optimistic that the Fed and the ECB have tools to stabilize their economies but it seems that some may also be positioning for some type of action from China, which could have a very meaningful impact on global growth,” said Steve Neimeth, portfolio manager at SunAmerica Asset Management. “Data from China over the past week have weakened materially and the probability that they launch some type of stimulus plan has increased dramatically.”

The Shanghai Composite closed down in August for the fourth-consecutive month, its longest monthly losing streak since 2004.

In the latest flurry of mobile-device launches, Nokiareleased two smartphones, which run on the latest Windows Phone operating software, in an effort to win back lost market share from the likes of Apple, Samsung and Google. Still, shares of the Finnish cellphone maker plunged nearly 15 percent following the announcement.

Yelp jumped after the company's CEO Jeremy Stoppleman said that the click-through rate on mobile web traffic is "significantly higher" than on PCs and that the company is positioned to “grow for a long time,” speaking at the Citi Technology Conference.

Meanwhile, Zynga advanced after the social gaming company released a sequelto its 2009 hit videogame, "Farmville," built entirely in 3-D.